Purity, Formulations and ChromatographyAQA GCSE Chemistry: Flashcards

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Define a pure substance in chemistry.

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Define a pure substance in chemistry.: A pure substance is a single element or compound that is not mixed with any other substance. It contains only one type of particle throughout.
What is a characteristic property of a pure substance?: A pure substance has specific, definite melting and boiling points. These points are constant and can be used to identify the substance.
Distinguish between a pure substance and a mixture.: A pure substance is a single element or compound with fixed melting and boiling points. A mixture contains two or more pure substances that have not chemically combined, and may have a range of melting and boiling points.
Define a formulation.: A formulation is a mixture that has been deliberately designed to have specific, useful properties. Examples include medicines, fuels, cleaning products, alloys, fertilisers and cosmetics.
Give three examples of formulations.: Any three of: medicines, fuels, cleaning products, alloys, fertilisers, cosmetics, paints, or shampoos.
What is paper chromatography used for?: Paper chromatography is used to separate the different components (substances) in a mixture based on their different solubilities in a solvent.
Explain how paper chromatography separates a mixture.: The mixture is placed on paper and a solvent is drawn up through the paper by capillary action. Different substances in the mixture travel different distances because they have different solubilities in the solvent. Substances that are more soluble travel further up the paper.
What does a chromatogram show?: A chromatogram shows the separated components of a mixture as a series of spots or bands. Each spot represents a different substance in the original mixture, positioned at different distances up the paper.
Write the formula for calculating an Rf value.: Rf = distance travelled by substance ÷ distance travelled by solvent Or: Rf = distance from baseline to substance spot ÷ distance from baseline to solvent front
What does Rf stand for?: Rf stands for 'retention factor' (or 'retardation factor'). It is a numerical value used to identify and compare substances.
What is the range of possible Rf values?: Rf values range from 0 to 1 (or 0 to 100 if expressed as a percentage). A value of 0 means the substance did not move; a value of 1 means it travelled as far as the solvent.
How are Rf values used to identify substances?: The Rf value of an unknown substance is calculated and compared with the Rf values of known standard substances run under the same chromatography conditions. If the Rf values match, the unknown substance is identified as that standard.
Why must chromatography be carried out under the same conditions to compare Rf values?: Rf values are only valid for comparison if the same solvent, paper, temperature and other conditions are used. Different conditions will produce different Rf values for the same substance.
In paper chromatography, what is the baseline?: The baseline is the line drawn in pencil (not pen) near the bottom of the paper where the mixture sample is initially placed before the solvent is added.
In paper chromatography, what is the solvent front?: The solvent front is the furthest point that the solvent reaches up the paper. The distance from the baseline to the solvent front is used in the Rf calculation.